Make Slot.NOT_KNOWN have a nice __repr__
Missed renaming DataSlot to just Slot in some spots.
Implement Slot.set_value_not_known in terms of Slot.set_value.
Update value_is_known to access value as a property, not a method.
Correct the names of the "not known" exception classes.
Exception.__init__ needs self as the first argument.
Correct typo of "slot" as "sloft".
Tests for the Slot class.